Environmental Sustainability Assessment of Hydrogen from Waste Polymers

Abstract

The rising demand for single-use polymers calls for alternative waste treatment pathways to ensure a circular economy. Here, we explore hydrogen production from waste polymer gasification (wPG) to reduce the environmental impacts of plastic incineration and landfilling while generating a valuable product. We assess the carbon footprint of 13 H2 production routes and their environmental sustainability relative to the planetary boundaries (PBs) defined for seven Earth-system processes, covering H2 from waste polymers (wP; polyethylene, polypropylene, and polystyrene), and a set of benchmark technologies including H2 from natural gas, biomass, and water splitting. Our results show that wPG coupled with carbon capture and storage (CCS) could reduce the climate change impact of fossil-based and most electrolytic routes. Moreover, due to the high price of wP, wPG would be more expensive than its fossil- and biomass-based analogs but cheaper than the electrolytic routes. The absolute environmental sustainability assessment (AESA) revealed that all pathways would transgress at least one downscaled PB, yet a portfolio was identified where the current global H2 demand could be met without transgressing any of the studied PBs, which indicates that H2 from plastics could play a role until chemical recycling technologies reach a sufficient maturity level.

Critical Review of Life Cycle Assessment of Hydrogen Production Pathways

“…In [24], a PEM stack driven by wind energy or, when unavailable, by the Dutch grid generated 1.79 kgCO2/kgH2. Approximately 50% of emissions are due to the production of offshore wind From the values reported in Table 3, the best and worst results are both associated with PEM electrolysis, with values of −101.12 kg CO2eq /kg H2 [32] and 41.4 kg CO2eq /kg H2 [27], respectively, in the case of using either bioenergy with CCS or the electric grid mix of Creta island. The authors refer to the fact that the negative value associated with PEM with bioenergy is due to the combination of a biogenic carbon energy source (wood chip) together with a CCS system.…”

“…The recourse of a concentrated solar power (CSP) plant led to a total GWP of 8.67 kg CO2 /kg H2 [30]. The only work that assesses the use of a hydropower or nuclear plant together with a PEM stack [32] reports GHG emissions equal to 3.25 kg CO2eq /kg H2 and 0.77 kg CO2eq /kg H2 , respectively. In the graph of Figure 3, the results of the GHG emission of PEM in relation to the energy sources adopted are reported.…”

“…Comparing the sole gasification with an integrated pyrolysis + gasification process, the latter scheme can drive to a GWP reduction of between 5% and 19%. GHG emissions for biomass gasification were found to be equal to 5.3 kg CO2eq /kg H2 (−15.4 kg CO2eq /kg H2 when coupled with CCS) [28], 1.1 kg CO2eq /kg H2 (−13.8 kg CO2eq /kg H2 with CCS) [32] and 1.5 kg CO2eq /kg H2 [40], respectively, when using southern yellow pine, poplar wood, or bio-oil coming from the pyrolysis of sawmill by-products. The latter case is the only that refers to a gate-to-gate approach, not including the energy and material consumption for three cases of cultivation and harvesting, since the feedstock is considered as a by-product of a different system.…”

“…The latter case is the only that refers to a gate-to-gate approach, not including the energy and material consumption for three cases of cultivation and harvesting, since the feedstock is considered as a by-product of a different system. Waste plastic gasification was studied by Salah et al [32] and by Williams et al [36] with different gasification agents. In [36], an equal share of waste plastic and biomass were gasified with either oxygen, air or steam, achieving a total GWP of 16 kg CO2eq /kg H2 , 24 kg CO2eq /kg H2 and 40 kg CO2eq /kg H2 , respectively.…”

Environmental impact of different scenarios for the pyrolysis of contaminated mixed plastic waste

“…Chemical recycling, on the other hand, makes it possible to produce products from plastic waste for a variety of uses, such as fuels and chemical feedstocks. 8,9 Chemical recycling is particularly advantageous for the treatment of contaminated and/or mixed plastic waste due to the economic and technical limitations of mechanical recycling.…”